In one minute, try to find as many differences between the two scenes as you can.

After you finish looking, have several friends do the same. What you'll find is that everyone has very different results. The number of differences found, the order that they were found, even which ones were found will be different.

Combine all the lists and throw out the duplicates, you'll have a complete list.

Software testing works exactly the same way. You're likely under a tight schedule, you find as many bugs as possible in the time you have, but someone else can come in, test the same code, and find additional bugs.

It's easy to fall into the trap of wanting to be solely responsible for testing your own piece of the software, but don't do it. There's too much to gain by having others help you out.

Watching how someone else approaches a problem is a great way to learn new testing techniques.